Background: Maternal birth trauma to the pelvic floor muscles is thought to be consequent to mechanical demands placed on these muscles during fetal delivery that exceed muscle physiological limits. The above is consistent with studies of striated limb muscles that identify hyperelongation of sarcomeres, the functional muscle units, as the primary cause of mechanical muscle injury and resultant muscle dysfunction. However, pelvic floor muscles' mechanical response to strains have not been examined at a tissue level. Furthermore, we have previously demonstrated that during pregnancy, rat pelvic floor muscles acquire structural and functional adaptations in preparation for delivery, which likely protect against mechanical muscle injury by attenuating the strain effect.
Objective: We sought to determine the mechanical impact of parturition-related strains on pelvic floor muscles' microstructure, and test the hypothesis that pregnancy-induced adaptations modulate muscle response to strains associated with vaginal delivery.
Study design: Three-month-old Sprague-Dawley late-pregnant (N = 20) and nonpregnant (N = 22) rats underwent vaginal distention, replicating fetal crowning, with variable distention volumes. Age-matched uninjured pregnant and nonpregnant rats served as respective controls. After sacrifice, pelvic floor muscles, which include coccygeus, iliocaudalis, and pubocaudalis, were fixed in situ and harvested for fiber and sarcomere length measurements. To ascertain the extent of physiological strains during spontaneous vaginal delivery, analogous measurements were obtained in intrapartum rats (N = 4) sacrificed during fetal delivery. Data were compared with repeated measures and 2-way analysis of variance, followed by pairwise comparisons, with significance set at P < .05.
Results: Gross anatomic changes were observed in the pelvic floor muscles following vaginal distention, particularly in the entheseal region of pubocaudalis, which appeared translucent. The above appearance resulted from dramatic stretch of the myofibers, as indicated by significantly longer fiber length compared to controls. Stretch ratios, calculated as fiber length after vaginal distention divided by baseline fiber length, increased gradually with increasing distention volume. Paralleling these macroscopic changes, vaginal distention resulted in acute and progressive increase in sarcomere length with rising distention volume. The magnitude of strain effect varied by muscle, with the greatest sarcomere elongation observed in coccygeus, followed by pubocaudalis, and a smaller increase in iliocaudalis, observed only at higher distention volumes. The average fetal rat volume approximated 3 mL. Pelvic floor muscle sarcomere lengths in pregnant animals undergoing vaginal distention with 3 mL were similar to intrapartum sarcomere lengths in all muscles (P > .4), supporting the validity of our experimental approach. Vaginal distention resulted in dramatically longer sarcomere lengths in nonpregnant compared to pregnant animals, especially in coccygeus and pubocaudalis (P < .0001), indicating significant attenuation of sarcomere elongation in the presence of pregnancy-induced adaptations in pelvic floor muscles.
Conclusion: Delivery-related strains lead to acute sarcomere elongation, a well-established cause of mechanical injury in skeletal muscles. Sarcomere hyperelongation resultant from mechanical strains is attenuated by pregnancy-induced adaptations acquired by the pelvic floor muscles prior to parturition.
Keywords: birth injury; pelvic floor muscles; pregnancy adaptations; rat.
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